1. Field of the Invention
This invention relates to an electro-optical device for converting a light input image into a projection image by its photoelectric effect and to the method of operating the device.
2. Description of the Prior Art
A device called a liquid crystal light valve has hitherto been known as said type of electro-optical device. As an example, European Patent Application Publication No. 0023796 (published Feb. 11, 1981) disclosed a liquid crystal light valve having a multilayer structure consisting of a liquid crystal layer, a dielectric mirror, and a photoresponsive layer sandwiched between two transparent electrodes. In such a liquid crystal light valve, the dielectric mirror is an element necessary to reflect projection light incident from the liquid crystal side, for preventing the light from reaching the photoconductive layer.
For the dielectric mirror, multilayer films are used which comprise, for example, ZnS, Na.sub.3 AlF.sub.6, MgF.sub.2, TiO.sub.2, or SiO.sub.2. In order to prepare a dielectric mirror capable of reflecting all the visible rays, at least about 15-layer lamination is necessary together with a precise control of thickness for each layer. This requires considerably advanced production techniques. Even with a dielectric mirror produced in such a way, perfect reflection of the projection light cannot be achieved in practice, needing additional preparation of a light-absorbing layer between the photoconductive layer and the dielectric mirror, to compensate the function of the mirror. Thus, such conventional liquid crystal light valves do not have desired sufficient functions, structures thereof are complicated, and production thereof is laborious and expensive.
In addition, these light valves utilize the birefringence effect, so that their control voltage range is narrow, that is, a considerably precise control of the operational voltage is required for obtaining projection images of the desired shade. Projection images uniformly shaded or multicolored are difficult to form with these light valves.
These light valves utilize the dynamic scattering effect, the phase transfer effect, or the field-controlled birefringence effect as an electrooptical effect of liquid crystals. In order to form projection images (those for a projection purpose) in the liquid crystal layer by utilizing such effect or to erase these images, the light valve needs to be provided with at least two transparent conductive films (electrodes) facing each other through the above-mentioned liquid crystal layer and photoconductive layer. An example of methods of forming and erasing projection images in such light valves is described in Appl. Phys. Letter, 34 450 (1979). According to this method, erasing a projection image is carried out by applying a voltage opposite in polarity to that for forming a projection image, between counter electrodes.
The formation of TV-compatible dynamic images by the above-mentioned conventional method involves the following disadvantage. Since the voltage application between the counter electrodes is continued until completion of forming a projection based on the input of one frame of TV-compatible light signals, the formation of a projection image based on the input of the next frame of light signals must be delayed to the moment when erasure of the preceeding frame of projection image is finished.
Another disadvantage of the conventional method is that it is inherently impossible for the method to address optional sites, for instance, an optional line or row, and form a projection image, based on the input of its light signals, by the voltage control.